Line interface circuits are provided on line interface boards and are controlled by a control processor on the line interface board, the control processor being common to a plurality of line interface circuits.
In order to be able to compensate for line resistance dependent attenuation on the line, the control processor needs information about the resistance of the line connected to the respective line interface circuit. A method of obtaining information about the line resistance when the current characteristic and supply voltage of the line interface circuit are known, comprises measuring the line voltage and calculating the line resistance from the measured line voltage.
To transfer the line voltage information to the control processor, either an external analog-to-digital converter or an internal analog-to-digital converter in the line interface circuit, is used today.
In the case of an external analog-to-digital converter, a separate output terminal is necessary on the line interface circuit to output an output voltage proportional to the line voltage.
With an internal analog-to-digital converter in the line interface circuit, at least one separate output terminal is needed on the line interface circuit for communication with the control processor.